The use of traction drive power transmissions which transmit power to a driven part through a traction drive mechanism in automobiles and industrial machinery has recently attracted attention, and in recent years there has been progress in research and development in this area. The traction drive mechanism is a power transmitting mechanism. Unlike conventional drive mechanisms it does not use any gears. This enables a reduction in vibration and noise as well as providing a smooth speed change in highspeed rotation. An important goal in the automobile industry is improvement in the fuel economy of automobiles. It has been suggested that if the traction drive is applied to the transmissions of automobiles in order to convert the transmission to the continuous variable-speed transmission fuel consumption can be reduced by at least 20% compared to conventional transmission systems. This is due to the fact that the drive can always be in the optimum fuel consumption region of an engine. Recent studies have resulted in the development of materials having high fatigue resistance and in a theoretical analysis of traction mechanisms. As regards the traction fluid the correlation of traction coefficients is gradually being understood on a level of the molecular structure of the components. The term "traction coefficient" as used herein is defined as the ratio of the tractional force which is caused by slipping at the contact points between rotators which are in contact with each other in a power transmission of the rolling friction type to the normal load.
The traction fluid is required to be comprised of a lubricating oil having a high traction coefficient. It is known that a traction fluid possessing a molecular structure having a naphthene ring exhibits a high performance. "Santotrack.RTM." manufactured by the Monsanto Chemical Company is a commercially available traction fluid. Japanese patent Publication No. 35763/1972 discloses di(cyclohexyl)alkane and dicyclohexane as traction fluids having a naphthene ring. This patent publication discloses that a fluid obtained by incorporating the above-mentioned alkane compound in perhydrogenated ( .alpha.-methyl)styrene polymer, hydrindane compound or the like has a high traction coefficient. Further, Japanese patent Laid Open No. 191797/1984 discloses a traction fluid containing an ester compound having a naphthene ring. It discloses that an ester obtained by the hydrogenation of the aromatic nucleus of dicyclohexyl cyclohexanedicarboxylate or dicyclohexyl phthalate is preferred as the traction fluid.
As mentioned above, in recent years there has been progress in the development of continuous variable-speed transmissions in the automobile industry. The higher the traction coefficient of the traction fluid the larger the allowable transmission force in the same device. This contributes to a reduction in the size of the entire device with a corresponding reduction in exhaust gas emission. Therefore, there is a need for a fluid having a traction coefficient which is as high as possible. However, the use of a currently commercially available traction fluid exhibiting the highest performance of all such currently commercially available fluids in such a traction drive device provides an unsatisfactory performance in respect to the traction coefficient. Furthermore, such a traction fluid is rather costly. The traction fluid which has been proposed in Japanese patent Publication No 35763/1971 contains .alpha.-methylstyrene polymer or its analogue as a component and, therefore, is also unsatisfactory as regards its performance and cost.
The present inventors have made extensive and intensive studies with a view to developing a traction fluid which not only exhibits a high traction coefficient but is also economical. As a result the present inventors have found that the combination of a specific amount of a hydrocarbonic polymer or esteric polymer and an ester compound or its derivative in which a cyclohexyl ring is connected to a linear-chain hydrocarbon through an ester linkage can provide a high-performance and economical base oil fluid. The present invention is based on this finding.